Update:
Still very loved and very spoiled every day. Have even installed an air conditioner in the van for him so he can continue to come to work on those few jobs where he can't get out and run around. He even asks to be let back into the van on hot days wile the boring humans do their work.
Awhile ago, I returned to the vet for yet more antibiotics. When I mentioned that I was worried the amoxicillin seemed to be getting less and less effective, the vet said she wasn't surprised since bacterial resistance to amoxicillin grows quickly. She gave us something else, just an antibiotic, but it must have been some way more serious stuff because the dog was basically cured. No symptoms returned for a long time.
He's got a little bit of breath starting up recently (not too bad) so we're going to go back for a professional assessment of his dental health (looks about the same to me, no signs of rot) and some more magic drugs. I'm feeling good about deciding not to risk anesthetising him simply for a cleaning.
I wish I could remember the name of the last antibiotic but there are probably so many that it wouldn't be of much use to others. Hope my $.02 here can help inform those facing similar problems with their pet - basically sound teeth, but gingivitis and bacterial problems at the gumline caused by tartar buildup.
Thanks for indulging my longwindedness and for your advices. You're good sports.